1. Field of the Invention
The present invention relates to a rotary coupling for torque transmission in response to a rotational speed difference between two relatively rotatable shafts.
2. Discussion of the Prior Art
A rotary coupling for torque transmission between input/output shafts is disclosed in Japanese Laid-Open Patent Publication 63-240429. As illustrated in FIG. 1, the rotary coupling according to the conventional art has a housing 21 which receives in its space section 40 a rotor 41 having two or three blades 42a extending in a radial direction. Further a high viscous fluid 45 is filled in the space section 40 to constitute a pressure generating mechanism together with the rotor 41.
When a relative rotation occurs between the housing 21 and a rotary shaft 22, the high viscous fluid 45 filled in the space section 40 is compulsorily displaced by the blades 42a. An internal pressure proportional to the relative rotation is then generated in the space section 40, whereby a piston is slid in the axial direction so as to press clutch means. Consequently, the rotational torque is transmitted from the input shaft to the output shaft.
The above-mentioned pressure generating mechanism of a blade type contains air of a few volume percent in the space section in consideration of the thermal expansion of the high viscous fluid. The air contained in the space section does not affect the characteristic in the torque transmission under normal conditions. On the contrary, under the special conditions that the relative rotational direction of the blades abruptly changes during torque transmission, the transmissive torque might be decreased temporarily due to the air. The detailed phenomenon is as follows.
When the blades 42a rotate in the forward direction (that is the clockwise direction) relative to the housing 21 in response to a positive rotational speed difference N between the housing 21 and the rotary shaft 22, as shown in FIG. 1, the air is gathered to areas E1 located at backward side of the blades 42a with respect to the rotational direction of the blades 42a. However, when a negative rotational speed difference--N occurs abruptly and the blades 42a relatively rotate in a reverse direction (that is the counter clockwise direction), the air in the areas E1 is moved to areas E2 located at backward side of the blades 42a with respect to the rotational direction of the blades 42a. Since the air moves through a long distance from the areas E1 to E2, the pressure generated by the pressure generating mechanism tends to be relatively low during the movement of the air.
As a result when the rotational direction of the blades 42a changes abruptly, the transmissive torque is decreased temporarily as illustrated by L2 in FIG. 2 wherein a curve L1 shows the change of the transmissive torque with respect to rotational speed difference. The torque transmission is thus delayed in such case.